1. Field of the Invention
The present invention relates to a nitride semiconductor structure and a semiconductor light emitting device, especially to a nitride semiconductor structure and a semiconductor light emitting device including a second type doped semiconductor layer with a high dopant concentration (larger than 5×1019 cm−3) and a small thickness (smaller than 30 nm) to improve a light-extraction efficiency and make the semiconductor light emitting device have a better light emitting efficiency.
2. Description of Related Art
Generally, a nitride light emitting diode is produced by forming a buffer layer on a substrate first. Then a n-type semiconductor layer, a light emitting layer and a p-type semiconductor layer are formed on the buffer layer in turn by epitaxial growth. Next use photolithography and etching processes to remove a part of the p-type semiconductor layer and a part of the light emitting layer until a part of the n-type semiconductor layer is exposed. Later a n-type electrode and a p-type electrode are respectively formed on the exposed n-type semiconductor layer and the p-type semiconductor layer. Thus a light emitting diode device is produced. The light emitting layer has a multiple quantum well (MQW) structure formed by a plurality of well layers and barrier layers disposed alternately. The band gap of the well layer is lower than that of the barrier layer so that electrons and holes are confined by each well layer of the MQW structure. Thus electrons and holes are respectively injected from the n-type semiconductor layer and the p-type semiconductor layer to be combined with each other in the well layers and photons are emitted.
The brightness of LED is determined by the internal quantum efficiency and a light-extraction efficiency. The internal quantum efficiency (IQE) is the ratio of electron hole pairs involved in radiation recombination to the injected electron hole pairs. The refractive index of air and GaN respectively is 1 and 2.4. According to total internal reflection equation, the critical angle of GaN LED that allows light to be emitted into air is about 24 degrees. Thus the light-extraction rate is about 4.34%. Due to total internal reflection of GaN and air, light emitting from LED is restricted inside the LED and the light-extraction rate is quite low. Thus many researches focus on improvement of the light-extraction efficiency. For example, one of the methods is to perform surface treatments on a p-type GaN layer for reducing the total internal reflection and further improving the light-extraction efficiency. The surface treatment includes surface roughening and changes of LED morphology. Another method is to separate the n-type GaN layer from the substrate and a rough structure is formed over the n-type GaN layer. Then the GaN semiconductor layer is attached to the substrate by glue for improving the light-extraction efficiency. However, the first method can only be used to treat an exposed p-type GaN semiconductor layer on top of the LED chip. Thus the improvement of the light-extraction efficiency has a certain limit. The process of the second method is quite complicated and the glue has a problem of poor heat dissipation. Therefore the light emitting efficiency of LED produced by the above two methods is unable to be increased effectively.
Moreover, the concentration of the dopant in the p-type GaN layer is unable to be increased effectively so that the resistance of the p-type GaN layer is quite large. Thus current is unable to be spread evenly in the p-type GaN layer when the current flows from metal electrodes to the GaN semiconductor layer. The uneven current spreading results in that the lighting area is confined under the metal electrodes (n-type electrode ad p-type electrode). The light emitting efficiency of LED is also decreased significantly.
In order to overcome the above shortcomings of the nitride semiconductor structure and the semiconductor light emitting device available now, there is a need to provide a novel nitride semiconductor structure and a new semiconductor light emitting device.